Study of the stereoselectivity of 2-azido-2-deoxygalactosyl donors: Relationship to the steric factors of glycosyl acceptors

Article abstract of DOI:10.1016/j.carres.2011.08.015

The stereoselectivity of a 2-azido-2-deoxygalactosyl (GalN₃) donor is found to strongly depend on the nature of the acceptors in glycosylation reactions. The order of the acceptor, the stereochemistry, and the configuration of the monosaccharid

Full text of DOI:10.1016/j.carres.2011.08.015

Carbohydrate Research 346 (2011) 2380–2383
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Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Study of the stereoselectivity of 2-azido-2-deoxygalactosyl donors:
relationship to the steric factors of glycosyl acceptors
⇑
Jane Kalikanda, Zhitao Li
Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2011
Received in revised form 8 August 2011
Accepted 12 August 2011
Available online 22 August 2011
The stereoselectivity of a 2-azido-2-deoxygalactosyl (GalN3) donor is found to strongly depend on the
nature of the acceptors in glycosylation reactions. The order of the acceptor, the stereochemistry, and
the configuration of the monosaccharide all affect the stereochemistry outcome. More reactive acceptors
are observed to favor b-products, while less reactive acceptors afford more
a-products.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Glycosylation
Stereoselectivity
Acceptor reactivity
1. Introduction
was also discovered to improve a-selectivity. Other factors, includ-
ing the acceptors, the leaving groups of donors and activation
methods, are also known to affect the stereoselectivity.19–21,15 In
this paper, we report our recent research on the correlation
between a-selectivity of a GalN3 donor and the structure of glyco-
syl acceptors, more specifically, the stereochemistry and the
Chemical synthesis provides one of the most important means
to access large quantity of structurally well-defined carbohydrate
compounds in homogenous form.1,2 However, synthesis of oligo-
saccharides is much more challenging than synthesis of other types
of biopolymers (like peptides and nucleotides), largely due to the
difficulties in controlling the stereoselectivity and regioselectivity.
The control of the stereochemistry (which is not present in cases of
peptide linkages and nucleotide linkages) is especially challenging
because of the complexity of the contributing factors to the stere-
oselectivity, such as the configuration of the glycosyl donor,3–8 the
structure of the leaving group,9 the reaction conditions, the reac-
tivity of the glycosyl acceptor,10,11 and the protecting groups on
the glycosyl donors. Among all glycosidic linkages, 1,2-cis-linkages
configuration of the acceptors.
2. Results and discussion
2.1. Switch of stereoselectivity of donor 1
During our research of the relationship between protecting
groups of glycosyl donors and the stereoselectivity, we accidentally
found that one of the donors we made (compound 1, Scheme 1)
showed totally opposite stereoselectivity in reactions with two dif-
ferent acceptors. In a reaction between donor 1 and acceptor 2, we
were surprised to see that only b-disaccharide (compound 3) was
isolated from the reaction, despite the non-participating nature
(like a-linkages in glucose or galactose like donors and b-linkages
in mannose like donors) are more difficult to form than the
1,2-trans-linkages, which are convenient to form in the presence
of participating neighboring groups. The
a-GalNAc linkage, for
example, has been such a linkage of great synthetic interest be-
of the azido group and the generally a-selective nature of this do-
cause of its widespread presence in biological systems.12,13 Among
nor in our previous study and another literature report.22 However,
all the methods people developed for efficient synthesis of a-Gal-
when a different acceptor (compound 4) was applied, under the
NAc linkages, 2-azido-2-deoxygalactosyl (GalN3) donors have been
particularly popular because of their convenience to access and to
transform to amino group.14–16 Our previous research on the
remote protecting group effect on the stereoselectivity of a series
of GalN3 donors suggest that acetyl groups at 3 and 4 positions
same reaction conditions, only
a disaccharide (compound 5) was
isolated, which is consistent with our previous observation and lit-
erature report. It is apparent that the stereoselectivity of donor 1
can switch depending on the nature of the acceptors, even though
2-azido donors are generally considered
a-selective. Acceptor
are critical for high a
-selectivity.17,18 Higher reaction temperature
dependence of stereoselectivity in glycosylation has been observed
and reported by several groups before, but the total inversion of
selectivity of a same glycosyl donor under that same reaction con-
ditions is still surprising.23–25
⇑
Corresponding author.
E-mail address: zli@binghamton.edu (Z. Li).
0008-6215/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2011.08.015

J. Kalikanda, Z. Li / Carbohydrate Research 346 (2011) 2380–2383
2381
OBn
O
BnO
HO
AcO
O
O
TMSOTf/DCM
-78°C
OMe
BnO
BnO
O
BnO
OBn
O
BnO
AcO
N3
BnO
BnO
BnO
OMe
3
2
N3
OBn
O
BnO
AcO
O
CCl3
NH
1
HO
Ph
O
N3
TMSOTf/DCM
-78°C
O
O
O
AcO
Ph
O
O
O
OMe
4
AcO
OMe
5
Scheme 1. Opposite stereoselectivity of donor 1 in reactions with two acceptors (2 and 4).
It is not obvious how this result can be explained. Based on a
generally accepted mechanism and our previous experience with
similar donors, we propose that there are two competing mecha-
nisms for the formation of disaccharides: an SN1-like mechanism
through an oxocarbenium intermediate and an SN2-like mecha-
nism through a glycosyl triflate intermediate. Through the SN1 like
mechanism, the stereoselectivity is closely related to the confor-
mation of the oxocarbenium ion intermediate.26 There are two
common conformations for oxocarbenium ions, 4H3 and 3H4 half
this unique reactivity of donor 1 may be used as an indicator of the
reactivity of corresponding acceptors in glycosylation reactions. If
we can test the stereoselectivity of various glycosyl acceptors, the
information gathered could be used as a reference as to the reactiv-
ity of acceptors and used for future prediction of their stereoselec-
tivity in other reactions.
2.2. Test of a series of glycosyl acceptors
chairs. 4H3 often favors formation of
a
-glycoside and 3H4 favors
We decided to first study how the configuration of the glycosyl
acceptors affect the reactivity and then the stereoselectivity of the
glycosylation reaction. Eleven more glycosyl acceptors (compound
6–16, Fig. 1) were therefore synthesized from commercially avail-
formation of b-glycoside.10 The preference of the oxocarbenium
is therefore the key factor in controlling the stereoselectivity. The
preference of the conformation of the oxocarbenium can be
affected by many factors.27,28 Oxocarbenium of galatose donors
has been reported to prefer 4H3 halfchair.29 In the case of donor
1, our previous study also suggested that 4H3 is the preferred con-
formation.17 The SN1 pathway would therefore most likely afford
able methyl-a-glycosides through either literature procedures or
modified procedures from literature (Fig. 1).33–43
All acceptors are protected with benzyl group, so that the steric
and electronic effect from protecting group could be normalized.
This group of acceptors includes three most common families of
monosaccharide building blocks: glucose, galactose and mannose.
It also covers all four possible positions of hydroxyl groups in each
monosaccharide. The idea was to test and see how the structure of
acceptor would affect the stereoselectivity and the relative reactiv-
ity. All these acceptors were then tested in reactions with donor 1
under the same reaction conditions. The products were isolated
and characterized using NMR with results summarized in Table 1.
The results are generally consistent with our hypothesis. For
example, primary acceptors, like 9 and 13, all favor b-products in
the reactions (1:4 and 1:10, respectively). At the same time, axial
a-product. The rate determining step of SN1 pathway should be
the rate of the oxocarbenium ion formation, which should be a
constant in these two reactions and independent of the reactivity
of the acceptor. The SN2 pathway, on the other hand, will involve
glycosyl triflate or oxocarbenium ion pair species as intermedi-
ate.30 The stereoselectivity will be related to the stereochemistry
of these intermediates and the activation energy of the SN2 pro-
cess. In the case of donor 1, our hypothesis is that it will favor b
product due to the higher population of a-glycosyl triflate interme-
diate. The rate determining step of the SN2 pathway should be the
bimolecular substitution step, which is dependent on the reactivity
of acceptor. Compound 2, as a primary acceptor, must have very
high nucleophilicity and the SN2 pathway is therefore much faster
than the SN1 pathway, which afforded only b product. Compound
4, as a secondary axial acceptor, has much lower reactivity in SN2
reaction and the rate of SN2 reaction is much lower than that of
SN1 pathway. The reaction of the compound therefore afforded
only a-product through the SN1 pathway. Based on this hypothesis,
the stereoselectivity of this reaction totally depends on the reactiv-
ity of the acceptors. More reactive acceptor should give more
b-product and less reactive acceptor should give more a-product.
The reactivity of acceptors can be affected by a lot of factors,
including the stereochemistry, the order of the alcohol, the protect-
ing groups and the hydrogen bonding.31 It is therefore often very
difficult to predict the reactivity of a glycosyl acceptor.32 There is
also lack of experimental method to determine the relative reactiv-
ity of glycosyl acceptors in glycosylations. Due to these difficulties,
dependency of stereoselectivity of acceptor reactivity has been hard
to understand. However, on the other hand, the stereoselectivity of
acceptors, like 10 and 16 all give only a-products. However, the sec-
ondary equatorial acceptors showed quite dramatic disparity in the
test reactions. In case of glucose acceptors, 2-OH (acceptor 8) gives
only
a-product. On the other hand, 3-OH (acceptor 7) is b selective
(1:3.4), while 4-OH (acceptor 6) shows only low
a
-selectivity
(1.8:1). If our theory stands, these results suggest that the order
of reactivity in glucose acceptor is 6-OH > 3-OH > 4-OH > 2-OH.
Based on a similar analysis, the order of reactivity of galactose
acceptors is determined to be 6-OH > 2-OH > 3-OH > 4-OH. In case
of mannose acceptors, the order is 6-OH > 3-OH > 4-OH > 2-OH.
The relative reactivity of these acceptors, especially those with sec-
ondary equatorial hydroxyl groups, is hard to explain. We believe
that it is associated with the relative energy of the transition states
of the SN2 reactions, which should be a combined result from the
steric interactions of the donor and the acceptor. The results could
reflect a kind of ‘matchness’ between the donor and the acceptor at
the transition states. These results will provide useful information
for understanding the relative reactivity of acceptors. It must be

2382
J. Kalikanda, Z. Li / Carbohydrate Research 346 (2011) 2380–2383
HO
BnO
BnO
BnO
BnO
HO
BnO
O
O
O
BnO
O
BnO
BnO
BnO
HO
BnO
HO
BnO
BnO
OMe
OMe
OMe
OMe
OMe
2
6
7
8
OH
O
OBn
OBn
O
OBn
O
BnO
BnO
BnO
HO
OH
O
BnO
BnO
BnO
BnO
HO
12
BnO
11
BnO
OMe
OMe
OMe
9
10
BnO
HO
HO
BnO
OBn
BnO
OBn
O
OBn
O
OH
O
O
BnO
BnO
BnO
HO
BnO
BnO
BnO
OMe
OMe
OMe
OMe
13
16
14
15
Figure 1.
Table 1
OBn
O
BnO
AcO
OBn
O
BnO
AcO
O
TMSOTf/DCM
-78°C
OBn
BnO
HO
O
O
O
N3
OMe
N3
O
AcO
O
CCl3
O
N3
OMe
NH
OMe
Product
Entry
Acceptor
Sugar
OH #
Yield
a:b ratio
1
2
3
4
5
6
7
8
9
2
6
7
8
9
10
11
12
13
14
15
16
Glc
Glc
Glc
Glc
Gal
Gal
Gal
Gal
Man
Man
Man
Man
6
4
3
2
6
4
3
2
6
4
3
2
3
17
18
19
20
21
22
23
24
25
26
27
98
56
53
68
75
63
65
90
90
81
82
93
b Only
1.8:1
1:3.4
a
Only
1:4
a
Only
3:1
1.3:1
1:10
1.2:1
1:4.7
10
11
12
a
Only
kept in mind that the relative reactivity of the secondary equatorial
acceptors is associated with this particular donor. Extra caution
must be applied when using this data for glycosylation with other
types of donors, especially donors with different configuration from
galactose. Further research including both experimental and theo-
retical approaches to understand the origin of this disparity in ste-
reoselectivity is underway. We are also developing protocols based
on these data for more efficient stereoselective synthesis of GalN3
disaccharides. The results will be reported in a timely manner.
could be a good reference for future study of glycosylation reaction
of similar acceptors.
4. Experimental
4.1. Materials and general methods
Unless otherwise noted, reagents and solvents were obtained
from commercial suppliers and were used without further purifi-
cation. TLC was performed on pre-coated aluminum plates (Silica
Gel F254). Spots were visualized by exposure to UV light or by
immersion in p-anisaldehyde solution followed by heating. All
NMR spectra were recorded on a 360 MHz spectrometer. All proton
NMR data were obtained at 360 MHz and all carbon NMR data
were obtained at 90 MHz. Proton and carbon chemical shifts are
reported in parts per million (ppm) using CDCl3 as an internal ref-
erence unless otherwise noted. Coupling constants (J) are reported
in hertz (Hz) and multiplicities are abbreviated as follows: singlet
(s), doublet (d), triplet (t), quartet (q), multiplet (m) and broadened
(br).
3. Conclusion
Our research demonstrated that the stereoselectivity of the
GalN3 donor (1) is directly associated with the structure of the
acceptor. Based on our hypothesis, this disparity is most likely
associated with the difference in reactivity of the acceptors derived
from the steric factors. These reactions also provide a unique way
to characterize the relative reactivity of different acceptors under
similar electronic environment. Even though we cannot explain
the relative reactivity explicitly, the data obtained in this research

J. Kalikanda, Z. Li / Carbohydrate Research 346 (2011) 2380–2383
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2383
4.2. General procedure for glycosylation
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activated molecular sieves (50 mg/mL solvent) in anhydrous
dichloromethane (5.0 mL/mmol donor) was stirred at room tem-
perature for 0.5 h. The reaction was cooled to ꢀ78 °C and TMSOTf
(0.15 equiv) was added followed by stirring the solution at ꢀ78 °C
for 15 min, then warming to room temperature for 15 min. It was
quenched by addition of Et3N, concentrated and purified using
flash column chromatography to give the products.
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8050.
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a/b product ratio
The determination of the ratio of the anomers was achieved in
two steps. First, the relative quantity of the a/b anomers was qual-
itatively determined by characteristic NMR signals, including 1H
13
NMR of the H10 and H30 and C NMR of C10. The ratio was then
determined quantitatively using the following diagnostic protons.
The identity of these diagnostic protons was established through
comparison of the integration with the characteristic protons.
Compound
Diagnostic protons
d
a
(ppm)
db (ppm)
17
18
20
22
23
24
25
26
2.05 (s, 3H, CH3, OAc)
3.29 (s, 3H, OCH3)
2.03 (s, 3H, CH3, OAc)
3.28 (s, 3H, OCH3)
2.08 (s, 3H, CH3, OAc)
2.06 (s, 3H, CH3, OAc)
2.06 (s, 3H, CH3, OAc)
2.04 (s, 3H, CH3, OAc)
3.37 (s, 3H, OCH3)
2.02 (s, 3H, CH3, OAc)
2.04 (s, 3H, CH3, OAc)
2.02 (s, 3H, CH3, OAc)
2.01 (s, 3H, CH3, OAc)
3.34 (s, 3H, OCH3)
3.35 (s, 3H, OCH3)
3.30 (s, 3H, OCH3)
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Acknowledgment
The financial support for the research is provided by Bingham-
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Supplementary data
Supplementary data (synthesis and characterization of accep-
tors 2, 6–16 and selected disaccharide products) associated with
this article can be found, in the online version, at doi:10.1016/
j.carres.2011.08.015.
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